1. Field of the Invention
The present invention relates to a head transfer apparatus for transferring a head for recording information on any appropriate information track of a recording medium, regenerating the recorded information from the information track or erasing the recorded information on the track.
2. Description of the Prior Art
Head transfer apparatus are widely known which include a guide means composed of a plain bearing and a guide cooperative to guide a movable unit with an element for recording information on any appropriate information track of a recording medium, regenerating the recorded information from the information track or erasing the recorded information of the track. Such a known guide means is simple in construction and can be manufactured at a low cost. However, the guide means involves a certain clearance between the bearing and the guide which then causes a wobbling motion between the bearing and the guide, hindering smooth movement of the movable unit. Another drawback associated with the known guide means is that due to the clearance, the bearing and the guide chatter each other when subjected to vibration or impact.
The plain bearing based on sliding action involves sliding friction so that it is subjected to a great bearing force imposed thereon by the weight of the movable unit. Since the weight of the movable unit continuously acts on an interface between the guide and the bearing, the bearing life is considerably reduced.
In order to overcome the drawbacks resulted from the clearance between the bearing and the guide, an improvement has been proposed as disclosed in Japanese Patent Laid-open Publication No. 62-47878 (hereinafter referred to as "first prior art"), wherein head transfer apparatus includes means for limiting the clearance between a plain bearing and a guide that are cooperative to guide movement of a movable guide unit having an objective lens for collecting a light beam onto a desired information track on the recording medium.
The clearance limiting means comprises a pair of burr-like annular resilient projections extending radially inwardly from opposite edges of an axial through-hole of the bearing. The annular projections are made of plastic and have an inside diameter slightly smaller than the outside diameter of a guide extending through the through-hole. The burr-like annular resilient projections fit over the periphery of the guide and resiliently support the latter under the resiliency acting radially inwardly toward a longitudinal axis of the guide, thereby eliminate the clearance between the through-hole in the bearing and the guide. The annular resilient projections can readily be fromed by injection molding, for example, into integral formation with the body of the bearing.
According to the first prior art, the bearing slidably supported on the guide is formed of synthetic resin and has the resilient projections for resiliently restricting the position of the guide relative to the bearing. With this construction, the movable unit is light in weight, inexpensive to manufacture and slidable smoothly without wobbling which would otherwise occurs due to the presence of a certain clearance between the bearing and the guide.
The guide means according to the first prior art is however still unsatisfactory in that a continuous operation of the head transfer apparatus results in a progressive wear of the plastic resilient projections which then produces a clearance between the bearing and the guide. The guide means involving the clearance is not reliable in operation.
Another type of conventional guide means used for guiding movement of a movable unit includes a ball bearing and a guide extending through a central hole of the ball bearing. The guide means includes a presser plate spring for urging the bearing against the guide to thereby control a clearance therebetween. The perpendicular pressure or bias imposed on the bearing varies with the force of the presser plate spring with the result that friction acting on a transfer mechanism is cause to fluctuate. Accordingly, the movable unit is transferred at non-uniform speeds, thus making it difficult to achieve an uniform access operation.
Japanese Patent Laid-open Publication No. 62-109267 (hereinafter referred to as "second prior art") discloses one solution to overcome the foregoing drawback which is resulted from the varying perpendicular pressure acting on the bearing when the clearance between the bearing and the guide is controlled. According to the disclosed solution, a head transfer apparatus is provided with a ball bearing for guiding movement of a movable unit, and means for exerting a constant perpendicular bias on the bearing to thereby control a clearance between the bearing and a guide in such a manner as to enable a uniform access operation.
The clearance controlling means according to the second prior art includes a magnetic substance disposed on a movable unit, and a permanent magnet disposed on the outside of the movable unit in confronting relation to the magnetic substance. Under the influence of the magnetic flux produced from the permanent magnet, the magnetic substance is caused to produce a magnetic force acting to impose a constant perpendicular pressure or bias on the bearing to keep the latter in contact with the guide while the bearing is rotating.
Since the perpendicular bias can be obtained the clearance controlling means which is held out of contact with the bearing and the guide, the head transfer apparatus of the second prior art does not involve undesirable fluctuation of the perpendicular pressure or bias.
However, since the permanent magnet is necessary in addition to the magnetic substance, the head transfer apparatus includes an increased number of components, and hence is costly to manufacture and large in size.